1. Field of the Invention
The invention relates to fused polymeric materials having improved physical properties. More specifically, the invention relates to fused granular polymeric materials having improved flexural modulus.
2. Discussion of the Background
Bonding of particles in a powder mass by molecular or atomic attraction in the solid state can be accomplished in a variety of ways, e.g., by means of the application of heat. The application of heat produces a strengthening of the powder mass and adhesion of the powder particles and normally results in densification of the material. Techniques such as sintering are generally conducted under conditions of increased pressure and temperature to effect the adhesion of the powder particles.
The technique of sintering has been used in powder metallurgy to effect consolidation of metal powders by the application of heat and pressure. During the sintering process, the strength and density of the powder mass increases while the porosity generally decreases. The grain structure of the metal particles undergoes changes and recrystallization and grain growth frequently occur. Many types of metal industrial parts are prepared by sintering, such as, for example, bearings, electrical components, magnets, and nuclear fuel elements.
Sintering has also been used in the formation of refractory ceramics by the sintering of aluminum oxide or titanium dioxide, for example. The fabrication of a product employing this technique may be accomplished by mixing the powder material with an organic binder, and placing the powder/binder mixture in a sintering mold. During sintering, the organic binder volatilizes and along with trapped gases is removed by diffusion or by the application of vacuum, giving a final sintered product with increased density. Principal concerns during the final stages of ceramic sintering include the development of optimum microstructure and the avoidance of rapid grain growth as well as the elimination of porosity. Conventional sintering of both ceramic and metals, therefore, involves substantial microcrystalline changes in the powder particles.
Sintering of organic polymers has been applied particularly to the sintering of polytetrafluoroethylene (PTFE) powders. PTFE may be sintered in electrical ovens at temperatures up to about 400.degree. C. by either free sintering or pressure sintering processes. A homogeneous structure is generally formed when a preformed article is heated to about 370.degree.-390.degree. C. By careful cooling, the crystallinity and hence the product properties may be controlled.
The fusing of granular thermoplastic polymers, e.g., by sintering, which have been molecularly oriented is a novel concept and the present inventor knows of no reference which discloses such a process. Processes are known, however, for orienting thermoplastic materials.
Many processes are known by which the properties of thermoplastic materials can be altered by orientation processes. For example, molecular orientation can be produced in thermoplastic drawn fibers, in axially oriented films, etc. by a variety of orientation methods. Such methods generally substantially increase the flexural modulus and tensile strength in the direction of orientation while at best maintaining standard or normal tensile strength and flexural modulus in the direction perpendicular to the orientation. Orientation in thermoplastic materials is only generated by specific commercial and industrial processes, and the type of orientation achieved is specific to the process used to produce the orientation. The resulting oriented materials, however, do not exhibit overall isotropic increases in flexural modulus and tensile strength. Orientation, and the benefits thereof in thermoplastic materials is, therefore, not generally fully achieved with engineering plastics.
A need continues to exist for a method of preparing isotropic materials having increased flexural modulus and tensile strength.